Coral Calcifications Insights from Inorganic Experiments and Coral Responses to Environmental Variables

Michael C. Holcomb, Ph.D., 2010
Anne Cohen and Daniel McCorkle, Co-Advisors

Coral calcification is examined using a laboratory model and living corals. In
the laboratory model, abiogenic aragonite formed at high saturation state (W>~20)
had a granular appearance and was enriched in trace elements, similar to centers of
calcification and dark bands in corals. Abiogenic aragonite formed fibrous crystals at lower
saturation states, similar to crystals which radiate out from centers of calcification.
These similarities suggest the calcifying environment of the coral experiences a range
of saturation states. To estimate when high or low saturation states occur within
the coral, living corals were stained, staining patterns suggest dark bands form between
dusk and dawn. A model is proposed in which daytime saturation state is limited by the
availability of CO2. To test the potential for photosynthesis to limit CO2
availability to calcification, corals were grown under altered CO2 and nutrient levels. Elevated
CO2 levels decreased calcification in zooxanthellate corals, however addition of
nutrients reduced the negative impact of CO2. This suggests nutrient availability may
limit photosynthesis under elevated pCO2 conditions. The effects of nutrients, CO2,
and temperature were further tested on both zooxanthellate and azooxanthellate coral
colonies. Unexpectedly, a gender difference was found in the effect of CO2 on
calcification.